Systems and methods for making structures defined by CNT pulp networks

ABSTRACT

Provided herein are products and methods for making structures having a body defined by a carbon nanotube (CNT) pulp network having a long-range connectivity exceeding a percolation threshold of the structure to permit electron transport throughout the structure, an active material dispersed within the body, and a binder material binding the active material to the CNT pulp network within the body.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 15/351,912, filed Nov. 15, 2016. The notedapplication is incorporated herein by reference.

FIELD

The present disclosure relates to compositions and methods for makingcarbon nanotube (CNT) pulp networks and particularly to CNT pulpnetworks defining structures.

BACKGROUND

Rechargeable Energy Storage Devices (ESD's), such as Lithium IonBatteries (LiB's) and Super-Capacitors (SC's) are widely used inelectronic devices. However, ESD's are generally rigid structures havingenergy storing active materials of limited thickness and capacity.Moreover, the active materials in LiB cathodes are generally metaloxides that have little or no inherent electrical conductivity. In orderto transport electrons throughout the cathode active material, aconductive additive must be employed. Incumbent technology employs someform of carbon black (CB) as the conductive additive, which can limitthe usable thickness of the cathode active material layer. Thickeractive material layers, in general, require more CB to achieve theneeded electrical conductivity. However, if the concentration of CBexceeds about 5% by weight, the material becomes mechanically unstable,and will mud-crack upon drying. This limits the thickness of the cathodelayer to less than about 100 microns, requiring many layers to achievethe needed capacity for a full battery. Each layer must have a separatorand current collector, which can take up space and add weight withoutcontributing to energy storage capacity. Having thicker active layerswould reduce the number of layers in the battery, and therefore thenumber of separators, thus leading to an increase in volumetric andgravimetric capacity of the overall battery cell.

Well separated, short (<100 microns in length) carbon nanotubes inpowder form have been used as conductive additives in LiB cathodes, andhave achieved percolation threshold for electron transport in the activematerial at about 3 times lower concentration than carbon black.However, these powdered CNTs did not impart improvements in mechanicalstrength.

SUMMARY

In some embodiments, a structure is provided. The structure includes abody defined by a network of interconnected carbon nanotube (CNT) pulp,the CNT pulp being provided in an amount sufficient to permit electrontransport throughout the structure. The structure also includes a bindermaterial dispersed within the CNT pulp network. The structure alsoincludes an active material distributed throughout the body for ionstorage.

In some embodiments, a method for forming a structure is provided. Themethod includes mixing a carbon nanotube (CNT) pulp with a binder, anactive material, and a solvent to form a dispersion. The method alsoincludes applying the dispersion to a substrate. The method alsoincludes curing the dispersion to form a structure having a CNT pulpnetwork formed therein, the CNT pulp being provided in an amountsufficient to permit electron transport throughout the structure via thenetwork.

In some embodiments, an energy storage device is provided. The energystorage device includes a housing. The energy storage device alsoincludes a first current collector positioned in the housing. The energystorage device also includes a first structure in electricalcommunication with the first current collector. The first structureincludes a first body defined by a first network of interconnectedcarbon nanotube (CNT) pulp, the CNT pulp being provided in an amountsufficient to permit electron transport throughout the first structure.The first structure also includes a first binder material dispersedwithin the first CNT pulp network. The first structure also includes afirst active material distributed throughout the first body for ionstorage.

The energy storage device also includes a second current collectorpositioned in the housing. The energy storage device also includes asecond structure in electrical communication with the second currentcollector.

The second structure includes a second body defined by a second networkof interconnected carbon nanotube (CNT) pulp, the CNT pulp beingprovided in an amount sufficient to permit electron transport throughoutthe second structure. The second structure also includes a second bindermaterial dispersed within the second CNT pulp network. The secondstructure also includes a second active material distributed throughoutthe second body for ion storage. The energy storage device also includesa separator interposed between the first structure and the secondstructure for inhibiting direct electrical contact between the first andsecond structures and for permitting ion passage between the first andsecond structures.

In some embodiments, a method for forming a nanoscale silicon layer onthe CNT pulp is provided. The method includes placing a quantity of CNTpulp in a Chemical Vapor Deposition (CVD) reactor. The method alsoincludes flowing silane gas over the CNT pulp within the CVD reactor.The method also includes heating the CNT pulp in order to coat the CNTpulp with a nanoscale layer of silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently disclosed embodiments will be further explained withreference to the attached drawings. The drawings shown are notnecessarily to scale, with emphasis instead generally being placed uponillustrating the principles of the presently disclosed embodiments.

FIG. 1 is a flowchart illustrating a method for forming a structuredefined by a CNT pulp network in accordance with various embodiments.

FIG. 2 is a diagram illustrating a system for direct collection ofwell-entangled networks of CNT material in accordance with variousembodiments.

FIG. 3 is a flowchart illustrating a method for forming a CNT pulp inaccordance with various embodiments.

FIG. 4 is a microscopic image of a portion of a structure defined by aCNT pulp network in accordance with various embodiments.

FIG. 5 is a block diagram illustrating an energy storage deviceincluding structures defined by CNT pulp networks in accordance withvarious embodiments.

FIG. 6 is a plot illustrating resistivity of various lithium ironphosphate cathode composites in accordance with various embodiments.

FIG. 7 is a plot illustrating cathode discharge capacity of variouscathode compositions in accordance with various embodiments.

FIG. 8 is a plot illustrating cathode discharge capacity of variouscathode loadings and compositions in accordance with variousembodiments.

FIG. 9A is an image of a conventional cathode undergoing flexure.

FIG. 9B is an image of a structure defined by a CNT pulp networkundergoing flexure in accordance with various embodiments.

FIG. 10 is a plot illustrating anode capacity of various anodecompositions in accordance with various embodiments.

FIG. 11 is a flowchart illustrating a method for forming nanoscalesilicon in accordance with various embodiments.

While the above-identified drawings set forth present disclosure, otherembodiments are also contemplated, as noted in the discussion. Thisdisclosure presents illustrative embodiments by way of representationand not limitation. Numerous other modifications and embodiments can bedevised by those skilled in the art which fall within the scope andspirit of the principles of the present disclosure.

DETAILED DESCRIPTION

In accordance with various embodiments, improved compositions andmethods for making structures having carbon nanotube (CNT) pulp networkstherein are provided. The compositions and methods, in accordance withvarious embodiments can include a structure having a body defined by aCNT pulp network, an active material, and a binder binding the activematerial and the CNT pulp network. The structure can be formed, in someembodiments, by curing a dispersion including a CNT pulp dispersedwithin an active material, a solvent, and a binder.

In some embodiments, the structures incorporate a interconnected networkformed from a plurality of bundles of well-entangled CNTs that achieve apercolation threshold for electron transport throughout the activematerial at a much lower concentration than either CB or powdered CNTs.In some embodiments, a well-entangled network of branched, bundled, andwell-dispersed CNT pulp containing long CNTs (>1 mm) can provide thenecessary electrical and ionic conductivity for a LiB at about 8 to 16times lower concentration than carbon black, and also provide themechanical support that enables thicker cathodes, flexible batteries,and advanced anode and cathode chemistries.

As used herein, percolation threshold refers to a concentration orweight percentage of conductive additive (such as carbon black or CNTpulp) sufficient to provide electrical and thermal conductivitythroughout the structure. That is, above the percolation threshold, theconductive additives are sufficiently connected to provide electricaland/or thermal conductivity throughout the structure. In the case ofelectrical conductivity, the interconnected conductive additives permitelectron transport throughout the active material. In contrast, belowthe percolation threshold, the long-range connectivity between theinterconnected conductive additives is insufficient to provideconductivity throughout the structure and thus electron transport, ifany, is limited to a small, localized portion of the active material.

The well-entangled network described in the present disclosure alsoimparts improved mechanical strength for enabling thicker cathodes,nano-scale silicon additions in anodes, and flexibility of thestructure. In some embodiments, the structures can include one or moreof a cathode, an anode, or an electrode of an Energy Storage Device ESD,such as a battery or capacitor.

Referring now to FIG. 1, a method 200 is provided for forming astructure in accordance with various embodiments. The method 200includes a step of mixing 201 a carbon nanotube (CNT) pulp with abinder, and active material, and a solvent to form a dispersion. Themethod 200 further includes a step of applying 203 the dispersion to asubstrate. The method 200 further includes a step of curing 205 thedispersion to form a structure having a CNT pulp network formed therein,the CNT pulp network having a long-range connectivity exceeding apercolation threshold of the structure.

The step of mixing 201 can include, for example, mixing a CNT pulp, abinder, an active material, and a solvent together using a high-shearmixer such as, for example, a Flacktek® dual asymmetric centrifugallaboratory mixer. In some embodiments, the step of mixing 201 caninvolve a single step. In some embodiments, the step of mixing 201 caninclude two steps. For example, in accordance with various embodiments,the step of mixing 201 can be performed by initially mixing the CNT pulpwith the binder and the solvent to form the dispersion and then addingthe active material and/or additional solvent to the dispersion andmixing again. In some embodiments, initially mixing the CNT pulp withthe binder and the solvent can be performed by operating the high-shearmixer at any appropriate speed for a period of time required to achievea substantially uniform distribution of the CNT pulp and the binderthroughout the dispersion. The dispersion of CNT pulp, binder, andsolvent can be prepared having any suitable viscosity, including, forexample, any viscosity of 3,000 centipoise or higher. In someembodiments this dispersion can exhibit a very high viscosity, forexample, about 20,000 to about 250,000 centipoise in order to achieve agood dispersion. Such high viscosity can prevent clumping of the CNTpulp within the dispersion during extended storage prior to curing.Therefore, in some embodiments, dispersions formed from the CNT pulp,the binder, and the solvent and having such high viscosity can becombined at a later time (e.g., after warehouse storage and/or shipment)with the active material and solvent, and mixed to complete thedispersion.

In some embodiments, when the active material is added and mixed, thedispersion can be diluted to produce a dispersion of viscosity suitablefor applying (203) and curing (205). In some embodiments, in order topromote formation of the CNT pulp network and provide a downstreamconsistency suitable for applying and curing to form the structure(e.g., as in the steps of applying 203 and curing 205), the dispersioncan be diluted to a dispersion having a viscosity of, for example, about3,000 to about 6,000 centipoise for applying and curing. In someembodiments, to preserve the substantially uniform distribution of CNTpulp and to preserve and promote formation of the well-entanglednetworks, the highly viscous dispersion can be serially diluted to thedesired viscosity. That is, in some embodiments, a portion of a totalamount of solvent required to achieve the desired viscosity (e.g.,between 3,000 to 6,000 centipoise) can be added and mixed with theactive material, then an additional portion of the solvent can be addedand mixed. This process can be repeated until the desired viscosity, andthus the desired dispersion, is achieved.

In some embodiments, the CNT pulp can include, CNTs. Presently, thereexist multiple processes and variations thereof for growing nanotubes,and forming yarns, sheets or cable structures made from these nanotubesto act as a source material for the pulp. These include: (1) ChemicalVapor Deposition (CVD), a common process that can occur at near ambientor at high pressures, and at temperatures above about 400° C., (2) ArcDischarge, a high temperature process that can give rise to tubes havinga high degree of perfection, and (3) Laser ablation.

In some embodiments, a CVD process or similar gas phase pyrolysisprocedure known in the industry can be used to generate the appropriatenanostructures, including carbon nanotubes. Growth temperatures for aCVD process can be comparatively low ranging, for instance, from about400° C. to about 1350° C. Carbon nanotubes (CNTs), both single wall(SWNT) or multiwall (MWNT), may be grown, in some embodiments, byexposing nanoscaled catalyst particles in the presence of reagentcarbon-containing gases (i.e., gaseous carbon source). In particular,the nanoscaled catalyst particles may be introduced into the reagentcarbon-containing gases, either by addition of existing particles or byin situ synthesis of the particles from a metal-organic precursor, oreven non-metallic catalysts. Although both SWNT and MWNT may be grown,in certain instances, SWNT may be selected due to their relativelyhigher growth rate and tendency to form rope-like structures, which mayoffer advantages in handling, thermal conductivity, electronicproperties, and strength.

The strength of the individual carbon nanotubes generated in connectionwith the present invention can be, for example, about 30 GPa or more.Strength, as should be noted, is sensitive to defects. However, theelastic modulus of the carbon nanotubes fabricated in the presentinvention may not be sensitive to defects and can vary from about 1 toabout 1.2 TPa. Moreover, the strain to failure of these nanotubes, whichgenerally can be a structure sensitive parameter, may range from a about10% to a maximum of about 25% in the present invention.

Furthermore, the nanotubes of the present invention can be provided withrelatively small diameter. In an embodiment of the present invention,the nanotubes fabricated in the present invention can be provided with adiameter in a range of from less than 1 nm to about 30 nm. It should beappreciated that the carbon nanotubes made in accordance with oneembodiment of the present invention may be extended in length (i.e.,long tubes) when compared to commercially available carbon nanotubes. Inan embodiment of the present invention, the nanotubes fabricated in thepresent invention can be provided with a length in the millimeter (mm)range.

It should be noted that although reference is made throughout theapplication to nanotubes synthesized from carbon, other compound(s),such as boron nitride, MoS2, or a combination thereof may be used in thesynthesis of nanotubes in connection with the present invention. Forinstance, it should be understood that boron nitride nanotubes may alsobe grown, but with different chemical precursors. In addition, it shouldbe noted that boron and/or nitrogen may also be used to reduceresistivity in individual carbon nanotubes. Furthermore, other methods,such as plasma CVD or the like can also be used to fabricate thenanotubes of the present invention.

In some embodiments, the CNT pulp can include, for example, CNT pulpformed as described with greater detail below with reference to FIG. 3.In some embodiments, the CNT pulp can be any CNT pulp capable of forminga three-dimensional CNT pulp network for providing a conductive aidhaving long-range electrical connectivity throughout the structure(i.e., exceeding a percolation threshold of the structure) whileenhancing mechanical properties and stability of the structure. Ingeneral, the CNT pulp can be made from any CNT sheet, CNT strip, CNTtape, bulk-collected CNTs, CNT yarns, any other suitable well-entangledCNT material, or combinations thereof.

In some embodiments, the CNT material, in accordance with variousembodiments, can be produced by Floating Catalyst Chemical VaporDeposition (FC-CVD) as described in U.S. Pat. No. 8,999,285, thecontents of which are incorporated herein in their entirety. The FC-CVDmethod of CNT production can lead to very long nanotubes (>100 microns)that become well-entangled while in the gas phase as they are beingcreated. As the CNT material exits the hot zone of the furnace, thenanotubes entangle, bundle and otherwise coalesce into and extendednetwork of interconnected and branching bundles that is not obtainableby other CNT production processes. In some embodiments, the extendednetwork of interconnected CNTs produced by FC-CVD is preserved throughthe pulping process, thus improving electrical and mechanical propertiesas compared to conventional carbon black and CNT powder.

In some embodiments, referring now to FIG. 2, CNT material can becollected from the FV-CVD reactor by a collection system 2000. Thesystem 2000, in some embodiments, can be coupled to a synthesis chamber2001. The synthesis chamber 2001, in general, includes an entrance end2001 a, into which reaction gases may be supplied, a hot zone 2002,where synthesis of extended length nanotubes may occur, and an exit end2001 b from which the products of the reaction, namely the extendedlength nanotubes and exhaust gases, may exit and be collected. In someembodiments, synthesis chamber 2001 may include a quartz tube 2003,extending through the hot zone 2002. Although illustrated generally inFIG. 2, it should be appreciated that other configurations may beemployed in the design of synthesis chamber 2001.

The system 2000, in some embodiments, includes a housing 2005. Thehousing 2005, as illustrated in FIG. 2, may be substantially airtight tominimize the release of potentially hazardous airborne particulates fromwithin the synthesis chamber 2001 into the environment, and to preventoxygen from entering into the system 2000 and reaching the synthesischamber 2001. In particular, the presence of oxygen within the synthesischamber 2001 can affect the integrity and compromise the production ofthe nanotubes.

System 2000 may also include an inlet 2005 a of the housing 2005 forengaging the exit end 2001 b of the synthesis chamber 2001 in asubstantially airtight manner. In some embodiments, as the CNT materialexits the synthesis chamber 2001, the nanotubes entangle, bundle andotherwise coalesce into and extended network of interconnected andbranching bundles. In some embodiments, these extended networks tend toform a hollow CNT “sock” similar in shape to a windsock inflated by abreeze. Thus, the CNTs can be collected within the housing 2005 from thesynthesis chamber 2001 by drawing the CNT sock 2007 onto a rotating meshdisk 2009 (e.g., by vacuum suction on a back side of the disk 2007) andremoving the CNTs from the rotating disk 2009 by a scalpel or “doctor”blade 2011, as shown in FIG. 2. In particular, as the CNT sock 2007 isdrawn onto the rotating mesh disk 2009, the CNT material forms a film onthe disk 2009, which the blade 2011 then scrapes off and severs as a newportion of the CNT sock 2007 is drawn onto the disk 2009. The CNTmaterial can then fall into or otherwise be transported to a collectionbin 2015 or other collection receptacle for subsequent pulping.

In some embodiments, the vacuum suction can be provided as a portion ofat least one gas exhaust 2013 through which gases and heat may leave thehousing 2005. Gas exiting from exhaust 2013, in an embodiment, may beallowed to pass through a liquid, such as water, or a filter to collectnanomaterials not gathered upstream of the exhaust 2007. In addition,the exhaust gas may be treated with a flame in order to de-energizevarious components of the exhaust gas, for instance, reactive hydrogenmay be oxidized to form water.

Although described above with reference to a collection system 2000having a rotating disk 2009 collection mechanism, it will be apparent inview of this disclosure that, in some embodiments, any technique forcollecting and removing the CNT material from the FC-CVD environmentwithout destroying the well-entangled CNT network can be used inaccordance with various embodiments. For example, collection of the CNTmaterial produced by FC-CVD, in some embodiments, can be performed byformation of CNT yarns or tows (e.g., by twisting collected CNTstogether) and/or CNT sheets as described in U.S. Pat. Nos. 7,993,620 and8,722,171, the contents of each of which are incorporated herein intheir entirety.

In some embodiments, the CNT material can initially include iron orother inclusions. In some embodiments, such inclusions are unwanted andcan be removed, preferably prior to pulping. For example, ironinclusions, in some embodiments, can be expunged from the CNT materialby heating the CNT material to high temperature (e.g., about 1800° C.)in an inert or reducing atmosphere. At such temperatures the iron can bedistilled out of the CNT material and re-solidified on a cooler surface.In some embodiments, such removal of inclusions can be performed, forexample, in a CVD reactor such as an FV-CVD reactor described above, orany CVD reactor described, for example, in U.S. Pat. Nos. 8,999,285 and7,993,620.

In some embodiments, inclusions such as, for example iron inclusions,can be removed by heating the CNT material to about 500° C. in air andtreated. In some embodiments, for example, the CNT material can beheated at the 500° C. in air for about two hours and then treated withmuriatic acid to remove iron inclusions.

In some embodiments, the CNT pulp can be formed from any suitable CNTmaterial, such as, for example, any CNT sheet, CNT strip, CNT tape,bulk-collected CNTs, CNT yarns, any CNT material described herein above,any other suitable well-entangled CNT material, or combinations thereof.Referring now to FIG. 3, a method 1100 is provided for forming a CNTpulp in accordance with various embodiments. The method 1100 includes astep of pulping 1101 by a pulping machine, one or more of a CNT sheet, aCNT strip, a CNT tape, bulk-collected CNT's, a CNT yarn, anywell-entangled CNT material, or combinations thereof to form a CNT pulp.The method also includes a step of grinding 1103 in a first grinder, atleast a portion of the CNT pulp. The method also includes a step ofdisaggregating 1105, in a second grinder, the CNT pulp.

The step of pulping 1101, in accordance with various embodiments, can beperformed by placing a strip or sheet or directly collected CNT materialinto a pulping machine and pulping the material to form a CNT pulp. Thepulping machine, in accordance with various embodiments, can include,for example, a Hollander beater, a conical refiner, a stamp mill, or anyother suitable mechanical pulping device, or combinations thereof.

In accordance with various embodiments, the CNT pulp can be tested toconfirm pulp particle size and then a user can determine whether or notto continue pulping. In some embodiments, the CNT pulp can be preparedfor grinding (e.g., as in the step of grinding 1103) by dewatering theCNT pulp to form, for example, a CNT press cake.

The CNT pulp, in some embodiments, can then be dried for furtherprocessing. Drying can be performed, for example, by air drying, ovendrying, vacuum oven drying, or by any other suitable drying process. Insome embodiments, the CNT pulp particles can be dried in an oven at atemperature from about 90° C. to about 110° C. for about 4 to about 12hours.

The step of grinding 1103, in accordance with various embodiments, canbe performed by using a grinder to break up the CNT pulp into CNT pulpparticles. In some embodiments, a particle size of the CNT pulp isunchanged by the grinder, which breaks up larger chunks of CNT pulp intoconstituent CNT pulp particles for subsequent drying. In someembodiments the grinder can include, for example, a coffee grinder, anindustrial burr mill, combinations thereof, or any other suitablegrinding device.

In some embodiments, the CNT pulp can be chemically modified and/orcoated to enhance an ionic conductivity of the CNT pulp. Such chemicalmodifications can include, for example, polysilazanes, polyureasilazane,conductive polymers, polyamine, polythiophene, infiltration withpolyamides, chemical modification to introduce carboxylate or aminefunctionalities, any modification suitable for enhancing ionicconductivity, or combinations thereof. In some embodiments, the chemicalmodification and/or coating can be performed after the step of grinding1103 but before the step of disaggregating 1105. However, it will beapparent in view of this disclosure that the chemical modificationand/or coating can be performed at any time including, for example,prior to pulping 1101, after pulping 1101 but before grinding 1103,after grinding 1103 but before disaggregation 1105, after disaggregation1105, or combinations thereof. It will further be apparent in view ofthis disclosure that, in some embodiments, the chemical modificationand/or coating can be performed in stages at different points throughoutthe pulping process and/or that multiple modifications and/or coatingscan be applied.

The step of disaggregating 1105, in a second grinder, the CNT pulp canbe performed by adding the dried CNT pulp to the second grinder (e.g., acoffee grinder, an industrial burr mill, combinations thereof, or anyother suitable grinding device). The step of disaggregating 1105, insome embodiments, also includes grinding the dried CNT pulp to break upany remaining clumps or agglomerates, thereby increasing a volume of theCNT pulp to form the CNT pulp. In some embodiments, the step ofdisaggregating 1105 can produce a CNT pulp having about 5 to about 15times the volume of the ground CNT pulp produced in the step of grinding1103 (i.e., the ground CNT pulp is about 5 to about 15 times more densethan the disaggregated CNT pulp). The step of disaggregating 1105 theCNT pulp advantageously provides greater surface area and betterdispersion of the CNT pulp. By reducing or eliminating agglomerations,the dispersion of the CNT pulp is improved and the risk of clumpingduring formation of the CNT pulp network is reduced. By contrast, if theCNT pulp is not well dispersed, the nanotubes will clump, and morematerial will be required to interconnect the active material particles,thereby reducing the amount of active material and thus reducing theperformance of the structure.

Referring again to FIG. 1, in some embodiments, the binder can include,for example, one or more of Polyvinylidene Fluoride (PVDF),Carboxymethyl Cellulose (CMC), Styrene Butadiene Rubber (SBR), orcombinations thereof. More generally, the binder can be any materialsuitable for binding the CNT pulp to the active material in the curedstructure.

In some embodiments, the solvent can include, for example, one or moreof n-Methyl-2-Pyrrolidone (NMP), propylene carbonate, water, ethanol,cyclohexylpyrrolidone (CHP), 1-benzyl-2-pyrrolidinone (NBenP), aniline,acetonitrile, dimethyl formamide, dichloromethane or combinationsthereof (e.g., a solvent being a solution of water and about 5% to about10% ethanol). The solvent, in some embodiments, can also include a pHbuffer for optimal ion conductivity and micro-structure in anodes madefrom aqueous dispersions. More generally, the solvent can include anysuitable fluid for dispersing a binder, CNT pulp, and active materialtherein.

The dispersion, in accordance with various embodiments, can include anyfluid mixture of CNT pulp, binder material, and solvent. In someembodiments, the dispersion can include about 0.1% to about 2% CNT pulpand about 0.4% to about 15% binder material dispersed in the solvent.For example, in some embodiments, the dispersion can include about 0.8%CNT pulp and about 4.8% binder dispersed in NMP solvent. In someembodiments, the dispersion can include about 1.0% CNT Pulp and about4.5% binder dispersed in a solvent solution of 5% ethanol and water.

The step of combining 203 can include, for example, combining an activematerial and additional solvent with the dispersion to form anotherdispersion. The active material, in accordance with various embodiments,can include using the high-shear mixer (or an additional high-shearmixer) to combine the additional solvent and the active material withthe dispersion. In accordance with various embodiments, the step ofcombining can be performed by operating the high-shear mixer at anyappropriate speed for a period of time required to achieve asubstantially uniform combination of the active material with thedispersion in the solvent. In some embodiments, the CNT pulp particlescan thereby bind and intertwine with the active material (e.g., as shownin FIG. 3 and described with greater detail below), thereby partiallyforming the three-dimensional CNT pulp network.

In some embodiments, the active material can include, for example, oneor more of Lithium iron phosphate (LFP), Nickel Manganese Cobalt oxides(NMC), Lithium Cobalt Oxide (LCO), Lithium Manganese Oxide (LMO),nanoscale silicon, graphite, or combinations thereof. More generally,the active material can be any material suitable for maintaining apositive or negative charge for storing electrical energy.

In some embodiments, the dispersion can include about 0.1% to about 2%CNT pulp, about 1% to about 5% binder material, and about 10% to about50% active material dispersed in the solvent. For example, in someembodiments, the dispersion can include about 0.25% CNT pulp, about 1.0%binder, and about 50% active material dispersed in NMP solvent. Inanother embodiment, the dispersion can include about 0.5% CNT pulp,about 2.2% binder, about 50% active material, about 5% ethanol in waterthat is buffered to about pH=3.

The step of applying 203 the dispersion to a substrate can be performed,for example, using one or more of a doctor blade, knife, trowel,dispenser, or combinations thereof. More generally, the step of applyingcan be performed using any suitable devices or systems capable ofdistributing the dispersion over a substrate.

The substrate, in accordance with various embodiments, can include anymaterial having a surface and chemically compatible with the dispersion.In some embodiments, for example, one or more of the substrate caninclude a current collector, a cathode collector, an anode collector, analuminum foil or plate, a copper foil or plate, a stainless steel foilor plate, wire or foil of any other suitable metal, carbon material inthe form of CNT sheet, tape, yarn, wire, graphene, graphite, or anycombination thereof. In some embodiments, the substrate can insteadinclude a Teflon or other sheet on which the dispersion can be cured andlater removed. For example, in some embodiments, an anode or cathodecollector can be integrated with the anode or cathode structure and,thus, the anode or cathode structure can be cured on the Teflon or otherremovable sheet and then removed for subsequent processing and use.

The step of curing 205 the dispersion to form a structure having a CNTpulp network formed therein can be performed, in accordance with variousembodiments, by one or more of air drying or heating the dispersion. Insome embodiments the dispersion is dried in air at 165C for two hours.

In accordance with various embodiments, once cured, the structure caninclude a body defined by the CNT pulp network, which can extendthree-dimensionally throughout the active material, and the bindermaterial binding the CNT pulp network with the active material. Forexample, referring now to FIG. 4, a scanning electron microscopy imageis provided of a structure 300, in accordance with various embodiments,comprising a CNT pulp and binder network 301, binding the activematerial 303 to the CNT pulp and binder network 301.

The CNT pulp network 301 can be formed, for example, from CNT pulpproduced as described in greater detail with reference to FIG. 3.Generally, the CNT pulp can be any CNT pulp capable of forming athree-dimensional CNT pulp network 301 for providing a conductive aidhaving long-range electrical connectivity throughout the structure(i.e., exceeding a percolation threshold of the structure) whileenhancing mechanical properties and stability of the structure.

The structure 300 depicted in the image of FIG. 4 includes a particularcomposition, by weight, of about 0.5% CNT pulp and about 3% PVDF binderin NMC active material, but structures in accordance with variousembodiments are not limited to this composition. For example, any of thestructures described herein can be used in accordance with variousembodiments. Furthermore, it will be apparent in view of this disclosurethat any composition having any combination of materials, any ratio ofmaterials, and/or any width, height, thickness, or shape can be used inaccordance with various embodiments provided that the CNT pulp network301 is present in an amount sufficient for exceeding a percolationthreshold (e.g., at about 0.5% CNT pulp) of the structure 300 andproviding enhanced material properties to the structure 300.

Referring now to FIG. 5, an energy storage device (ESD) 100, inaccordance with various embodiments, includes a first current collector102 associated with a first active layer 101 and a second currentcollector 104 associated with a second active layer 103, and a separator105 interposed therebetween.

The first and second current collectors 102, 104, in accordance withvarious embodiments can include aluminum foil, copper foil, stainlesssteel foil, wire or foil of any other suitable metal, carbon material inthe form of CNT sheet, tape, yarn, wire, graphene, graphite, or anycombination thereof. In some embodiments a distinctly separate currentcollector may not be required and one or both of the first and secondcurrent collectors 102, 104 can instead be integrated into the first orsecond active layer 101, 103.

In some embodiments, the first active layer 101 can be associated withcurrent collector 102. The first active layer 101 can include, forexample, an active material, binder, and a CNT network. In someembodiments, the first active layer can also include one or moreperformance enhancing materials. Performance enhancing materials, insome embodiments, may include, but are not limited to, carbon black,graphite, graphene, polymers, powder nanotubes, or any combinationthereof. In some embodiments the first active layer 101 can be a batterycathode, and the active material can include, but is not limited to,lithium iron phosphate (LFP), Lithium Cobalt Oxide (LCO), NickleManganese Cobalt Oxides (NMC), Sulfur, Encapsulated Sulfur, polymers,any other materials that can store charge transport ions, orcombinations thereof.

In some embodiments the first active layer 101 can be an electrode in asuper-capacitor or pseudo-capacitor, and the active material can includeof electrically conductive porous material, for example graphite,graphene, carbon fibers, carbon nanotubes, or any combination thereof,as well as materials exhibiting redox behavior, such as transition-metaloxides that may include but are not limited to Ruthenium Oxide, IridiumOxide, or Manganese Oxide, or combinations thereof.

In some embodiments, the binder can include, but is not limited to,Polyvinylidene Fluoride (PVDF), Styrene Butadiene Rubber (SBR),Carboxymethyl Cellulose (CMC), or other suitable soluble or dispersiblepolymers, or combinations thereof.

In some embodiments, the separator 105 can prevent or inhibit directelectrical contact between the two current collectors 102, 104, but canpermit passage of appropriate ions. The separator, in some embodiments,can include, but is not limited to, porous polyethylene (PE), and porouspolypropylene (PP), nylon, fiberglass, boron nitride nanotubes, orcombinations thereof.

In some embodiments, the second active layer 103 can be associated withthe second current collector 104. The second active layer 103 caninclude an active material, binder, and dispersed CNT network. In someembodiments, the second active layer 103 can also include performanceenhancing materials. In some embodiments the second active layer 103 caninclude a battery anode and the active material can include, but is notlimited to, graphite, silicon, gallium, Tin Oxide, Iron Oxide, Titaniumoxide, or any combination thereof. The binder could be, for example,Polyvinylidene Fluoride (PVDF), Styrene Butadiene Rubber (SBR),Carboxymethyl Cellulose (CMC), or other suitable dispersible polymers,or combinations thereof. In some embodiments the second active layer 103can include an electrode in a super-capacitor or pseudo-capacitor, andthe active material can include an electrically conductive porousmaterial such as, for example, graphite, graphene, carbon fibers, carbonnanotubes, or any combination thereof.

In some embodiments, the first active layer 101 can include a structureconstructed according to the methods described above with reference toFIG. 1, the first active layer 101 having a composition by weight ofabout 0.5% to about 20% CNT pulp, for example, about 1.0% to about 2.0%CNT pulp, about 2% to about 50% binder, for example, about 2% to about5% binder, and about 30% to about 97.5% active material. For example, insome embodiments, the first active layer 101 can have a composition byweight of about 0.5% CNT pulp, 3% PVDF (binder), and about 96.5% activematerial.

In some embodiments, the second active layer 103 can include a structureconstructed according to the methods described above with reference toFIG. 1, the second active layer 103 having a composition by weight ofabout 0.5% to about 2% CNT pulp, about 2% to about 5% binder, and about10% to about 95% nanoscale silicon. In some embodiments, the secondactive layer 103 can further include graphite, wherein the second activelayer 103 includes about 7.5% to about 87.5% graphite. For example, insome embodiments, the second active layer 103 can have a composition byweight of about 1% CNT pulp, about 4.5% CMC (binder), about 10%nanoscale silicon, and about 84.5% graphite.

FIG. 6 is a plot illustrating resistivity in ohm-centimeters of variouslithium iron phosphate cathode composites in accordance with variousembodiments. In particular, FIG. 6 compares a resistivity of aconventional cathode composite having about 1% carbon black by weightdispersed in lithium iron phosphate (LFP) active material, conventionalcathode composites having about 0.5% and about 0.9% CNT powder dispersedin LFP, and a cathode composite having about 0.1% to about 0.6% CNT pulpdispersed in LFP in accordance with the present disclosure. As shown inFIG. 6, the about 0.6% CNT pulp cathode composite exhibits substantiallyreduced resistivity (e.g., about 1 ohm-cm as compared to about 10 ohm-cmfor CNT powder and about 30 ohm-cm for carbon black) at lowerconcentrations (about 0.6% by weight as compared to about 0.9% for CNTpowder and 1% for carbon black) than the conventional carbon black andpowder CNT composites. Advantageously, reduced resistivity permitsfaster charging and discharging of the cathode and the lowerconcentration of conductive aid permits additional active material to beincluded in the cathode, thereby increasing cathode capacity.

In some embodiments, the CNT pulp forming the CNT pulp network canadvantageously exceed a percolation threshold of the structure. FIG. 7is a plot illustrating cathode capacity in mAh/g of variousNickel-Manganese-Cobalt (NMC) cathode compositions in accordance withvarious embodiments. In particular, FIG. 7 illustrates the cathodecapacity at various cathode discharge rates for a conventional compositeof 4% carbon black dispersed in NMC, a composite of about 0.25% CNT pulpin NMC, a composite of about 0.5% CNT pulp in NMC, and for a compositeof about 0.75% CNT pulp in NMC. As shown in FIG. 7, the about 0.25% CNTpulp composite does not achieve a complete percolation threshold andthus exhibits a relatively low capacity of about 50 mAh/g at a dischargerate of 2 C. However, at lower discharge rates such as C/2 and C/10, theabout 0.25% CNT pulp composite is able to match the performance of theincumbent 4% carbon black technology. As further shown in FIG. 7, eachof the other composites exhibits a similar capacity of about 110 mAh/gat a discharge rate of 2 C but the about 0.5% CNT pulp composite and theabout 0.75% CNT pulp composite exhibit a higher capacity (about 140 mAhas compared to about 120 mAh for the carbon black) at a lower dischargerates such as C/10. Thus, as shown in FIG. 7, structures formed with CNTpulp networks can achieve a percolation threshold at an 8× lowerconcentration (i.e., about 0.5% vs. about 4%) of conductive additive ascompared to conventional carbon black compositions for a discharge rateof 2 C (high power applications) For lower power applications a 16×lower concentration is adequate.

In some embodiments, because the CNT pulp network imparts improvedmechanical properties and lower resistivity to the cathode, cathodeswith higher loading of active material can be formed. FIG. 8 is a plotillustrating cathode capacity in mAh/g of various cathode loadings asindicated by cathode material density in milligrams per squarecentimeter (mg/sq cm) in accordance with various embodiments. Inparticular, FIG. 8 illustrates the cathode capacity at various cathodedischarge rates for a conventional composite of about 2.5% carbon blackand about 1.5% graphite dispersed in NMC having an active materialloading of about 21 mg/sq cm, a composite of about 0.5% CNT pulp in NMChaving an active material loading of about 12 mg/sq, and a composite ofabout 0.5% CNT pulp in NMC having an active material loading of about 21mg/sq. As shown in FIG. 8, due to the greater thickness of theconventional composite of about 2.5% carbon black and about 1.5%graphite, the conventional composite breaks down at the higher dischargerate of 2 C to a cathode capacity of about 40 mAh/g. By contrast, thethicker about 0.5% CNT pulp composite exhibits cathode capacity largelyconsistent with the thinner about 0.5% CNT pulp composite (about 110mAh/g at discharge rate 2 C as compared to about 120 mAh/g for the about12 mg/sq cm composite).

The improvement in performance of the cathode occurs because the CNTpulp network enhances the electrical conductivity as well as themechanical stability of the active material layer. Cathode activematerials (AM) are inherently non-conductive, and need a conductiveadditive (CA) to transport charge into and out of the AM. ConventionalCA technology is carbon black (CB). However, CB does not impart anymechanical strength to the material, therefore, as the AM gets thicker,the cathode becomes less mechanically stable, because more CB is neededto reach percolation threshold, and at some point the cathode activelayer falls apart. In tension with this concern, it is desirable inbattery functionality to make the active layer as thick as possible toreduce the volume in the cell taken up by non-active material, such asseparators and current collectors. However, increasing the quantity ofCB (e.g., from about 4% to about 5% CB or due to increased thickness)dramatically degrades the tensile strength of the electrode to nearlyzero. At higher than about 5% CB concentration, the cathode materialmud-cracks upon drying. Advantageously, the CNT pulp network disclosedherein not only imparts better electrical conductivity and thus requiresless conductive additive, but also enhances mechanical stability.Therefore, the CNT pulp network of the present disclosure permitsthicker cathodes without mechanical and electrical breakdown of thecathode.

Referring now to FIGS. 9A-9B, the enhanced mechanical stability impartedby the CNT pulp network further permits greater flexibility of thecathode and anode and, thus, any overall battery using such anodes andcathodes. Battery flexibility can be helpful for a wide range ofapplications, including wearable electronics, and personal computers.However, to obtain true flexibility in a battery the anode and thecathode must be mechanically robust and flexible. FIG. 9A is an image ofa conventional cathode having about 5% carbon black additive in LFP andFIG. 9B is an image of a cathode structure of the present disclosurehaving about 1% CNT pulp in LFP. As shown in FIG. 9A, the conventionalcathode cracks when wrapped around a 2.5 cm diameter dowel. By contrast,as shown in FIG. 9B, the about 1% CNT pulp cathode does not crack, evenwhen wrapped around a 0.32 cm dowel. It will be apparent in view of thisdisclosure that any other composition including CNT pulp can be used inaccordance with various embodiments. Generally, an increased percentageof CNT and binder can provide additional flexibility, although therewill likely be a trade-off between flexibility and capacity. However, inall compositions, the inclusion of CNT pulp material can impartflexibility and strength to battery active material that cannot beobtained by conventional carbon black or CNT powder additives.

In some embodiments, anodes can be provided having a CNT pulp networkextending therethrough. FIG. 10 is a plot illustrating anode capacity inmAh/g of various anode compositions over time in accordance with variousembodiments. In particular, FIG. 10 illustrates anode capacity as afunction of charge/discharge cycles for a conventional composite ofabout 10% nanoscale silicon (Si) in graphite and a composite of about10% nanoscale silicon and about 1% CNT pulp in graphite in accordancewith various embodiments as juxtaposed with the maximum theoreticalcapacities of an all-graphite anode and a about 10% silicon nanomaterialanode. The theoretical capacity of the about 10% Si anode is calculatedbased on an assumed maximum Li—Si stoichiometry of Li₁₅Si₄. As shown inFIG. 10, the anode including the about 1% CNT pulp outperforms thecomposite without CNT pulp by maintaining a higher anode capacity overall 11 test cycles and even approaches the theoretical maximum anodecapacity for an about 10% silicon anode at the second cycle. Thisfurther illustrates that additions of CNT pulp can be used to enhanceboth the electrical and mechanical properties of a coating even whenincluding large surface area nanoparticles such as Silicon.

By way of background, silicon is an attractive material for storinglithium metal in the anode of an LiB due to its high theoreticalcapacity: 4200 mAh/g for Li₂₂Si₅, or 3572 mAh/g for Li₁₅Si₄, compared toconventional anode technology (i.e., Graphite) with a theoreticalcapacity of only 372 mAh/g. However, one persistent issue with thesilicon addition to LiB anodes is that the capacity fades withcharge/discharge cycles. The fading is due to cracking/pulverization ofthe Si due to the 3× to 4× expansion/contraction duringlithiation/delithiation. This cracking does not occur with nanoscale Si,defined as silicon that is less than about 50 nanometers in diameter orthickness. However, production of nanoscale Si is typically veryexpensive, very difficult, or both, and creates challenges due to itshigh surface area in incorporating them into a coating that is bothcoat-able and subsequently mechanically robust. The fading seen incapacity with cycling seen in FIG. 10 is believed to be due to thepresence of a significant fraction of Si particles greater than about 50nm.

Referring now to FIG. 11, a chemical vapor deposition (CVD) method 1200is provided for forming a nanoscale silicon material. The methodincludes a step of placing 1201 a quantity of CNT pulp in a CVD reactor.The method also includes a step 1203 of flowing silane gas over the CNTpulp within the CVD reactor. The method also includes a step 1205 ofheating the CVD reactor to coat the CNT pulp with a nanoscale layer ofsilicon.

The step of placing 1201 the quantity of CNT pulp in the CVD reactor canbe performed, for example, by placing a quantity of a CNT pulp producedas described hereinabove or a quantity of another CNT nanoparticle intoa CVD reactor. The CVD reactor, in accordance with various embodiments,can include one or more of a cold-wall fluidized bed reactor, a hot-wallresistively heated furnace, or combinations thereof.

The step of flowing 1203 silane gas over the CNT pulp within the CVDreactor can be performed, for example, by providing a flow (e.g., via acompressed gas line, a gas bottle, or any other suitable gas flowingmechanism) of silane gas to the CVD reactor. In some embodiments, thestep of flowing 1203 can also include flowing the silane gas along withone or more additional gases in a mixture. For example, in someembodiments, a mixture of hydrogen, silane, and argon can be flowed overthe CNT pulp within the CVD reactor.

The step of heating 1205 heating the CVD reactor to coat the CNT pulpwith a nanoscale layer of silicon can be performed, for example, byheating a cold-wall quartz furnace by inductive heating or microwaveheating or by heating a hot-wall resistively heated furnace byresistively heating the reactor. It will be appreciated in view of thisdisclosure that, in accordance with various embodiments, the desiredcoating thickness and uniformity can be controlled by varying the pulptemperature, furnace pressure, and input gas composition and flow rate.

Various aspects of the present disclosure may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments. Also, thephraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for the use of the ordinalterm) to distinguish the claim elements.

What is claimed is:
 1. A method for forming a nanoscale silicon layer onCNT pulp comprising: placing a quantity of the CNT pulp in a ChemicalVapor Deposition (CVD) reactor; flowing silane gas over the CNT pulpwithin the CVD reactor; and heating the CNT pulp in order to coat theCNT pulp with a nanoscale layer of silicon wherein the CNT pulpcomprises a 3-dimensional extended network of interconnected carbonnanotubes each having a length of more than 1 mm.
 2. The method of claim1, further comprising flowing at least one of argon gas or hydrogen gaswith the silane gas over the CNT pulp within the CVD reactor.
 3. Themethod of claim 1, wherein the nanoscale layer of silicon coated on theCNT pulp is less than 50 nanometers thick.
 4. The method of claim 1,wherein the CVD reactor is a hot-wall resistively heated furnace and thestep of heating further comprises resistively heating the CVD reactor.5. The method of claim 1, wherein the CVD reactor is a cold-wallfluidized bed reactor and the step of heating the CNT pulp furthercomprises at least one of inductive heating or microwave heating.